Telephone systems of various sizes are also of various corresponding architectures. For example, central exchanges are usually large and characteristically are designed to include concentration of telephone lines in switching networks operated by a central controller complimented by various peripheral controllers. In the interest of extreme reliability, the central controller, switching network, and even some of the peripheral controllers are duplicated to achieve functional reliability through redundancy. Extensive maintenance software is provided to manage these elements so that in spite of inevitable randomly occurring failures or malfunctions, service to telephone subscribers continues to be substantially uninterrupted. In contrast, private branch exchanges (PBXs) tend to be smaller and more cost sensitive. PBX architectures are usually similar to that of central exchanges, however, with less duplication and redundancy and consequently less functional reliability. Advantages of the typical time division multiplex, pulse code modulation switching exchange are many, however one of the outstanding advantages is that of being adaptable to carry voice and or data signals typically at a base rate of 64 Kb/s, or for example in accordance with recent integrated service digital network (ISDN) recommended standards. Another advantage, exemplified in a typical modern telephone exchange, is that a plurality of directory or extension numbers may appear at any one telephone set while only a single telephone line actually connects the telephone set to the exchange.
Some examples of very small telephone systems are generally referred to as key telephone systems. Traditionally a key telephone system is provided by extensive telephone line and control lead wiring between key telephone sets. Each key telephone line extends to a telephone exchange. Each of the telephone sets includes a plurality of push button switches or keys, each for connecting the telephone set to a particular telephone line among a plurality of telephone lines routed to the key telephone set. The switching function of line selection is mechanically provided and distributed among the key telephone sets. Any features in addition to plain ordinary telephone service (POTS) must be added on a per line basis. The primary advantage of these systems is economy with small size. However, if such a system is required to expand along with the organization it serves, over a time it eventually becomes more expensive on a per line and feature basis than a private branch exchange would be. Key telephone systems are also characteristically of the analog signal type, and therefore are impractical to interface with an ISDN as will likely be desired by business customers in the near future.